1. Field of the Invention
The invention relates to a ceramic carrier, in particular to an Al2O3 carrier.
2. Discussion of the Related Art
A carrier of this type is known, for example, from JP 59 065 216 A. Said carrier is coated with a thin-film structure made of platinum and is used as a flow sensor for flow measurement.
Sensors with the same design principle are used as temperature measurement sensors in exhaust gas sensor systems. These are installed upstream of diesel soot particle filters, for example, in order to sense the exhaust gas temperature for regeneration of the filters. In this respect, the platinum thin-film sensors are exposed to severe fluctuating thermal loads, which have to be taken into consideration when designing the sensors with a view to the service life required in the automotive industry. The same applies to the use of platinum thin-film sensors in the automotive industry for monitoring the state of engine oil, the tribological properties of which depend greatly on the heating. For determining the state of the engine oil, the sum of the thermal loads is a crucial measured variable ascertained by platinum temperature sensors. In this respect, the sensors are exposed to many temperature change cycles, severe vibrational loading and to corrosive attack by the measurement medium.
Since the electrical resistance of a platinum sensor changes in an exactly defined manner with the temperature, the avoidance of measurement errors depends on suppressing as far as possible other influencing variables which alter the electrical resistance. In the case of severe fluctuating thermal loads, this problem arises when different materials are paired, as is the case for a ceramic carrier with a platinum thin-film structure. The different materials have different coefficients of thermal expansion, this also being referred to as a mismatch. In the case of fluctuating thermal loads, the different thermal properties of the materials lead to the plastic deformation of the platinum structures and to the migration of dislocations in the microstructure. The material properties are changed as a result. This leads to resistance drifts, i.e. undesired, mechanically induced changes in the resistance. Given severe mechanical stresses in the platinum structures, the latter can even be damaged or interrupted.
To date, attempts have been made to counter this problem by using material pairings which have similar coefficients of thermal expansion. By way of example, ceramic carriers made of zirconium oxide are used in combination with platinum thin-film structures. However, these have the disadvantage that the components structured in this way, in the event of further mechanical installation on Al2O3 ceramic hybrid carriers or modules, crack and are destroyed at the latest during cooling as a result of the higher level of expansion.
Another route is pursued in the prior art cited in the introduction. There, an attempt is made to reduce thermally induced stresses using a glass layer between the carrier and the platinum thin film. A sensor with such a structure does not satisfy the high demands made in respect of the stability and durability of thin-film sensors in the automotive industry.